As such a type of conventional actuator, there is an actuator disclosed in Japanese Patent Application Laid-Open Publication No. 11-170133. The conventional art shown in FIGS. 1 to 4 thereof is configured as follows.
A second piston for low load is inserted into a cylindrical hole of a first piston for high load which is inserted into a housing, and an output portion projects from the second piston to above the housing. A pressure fluid chamber is formed between an upper end wall of the housing and the first piston and the second piston. In addition, a first spring for high load is mounted between a lower end wall of the housing and the first piston, and a second spring for low load is mounted between a lower portion of the first piston and the second piston. Furthermore, a ball-type switching mechanism is disposed within a lower portion of the housing. The switching mechanism is configured to connect the first piston selectively to the lower end wall of the housing or the second piston.
The foregoing describes Japanese Patent Application Laid-Open Publication No. 11-170133
In the above-described conventional art, when a pressure oil supplied into the pressure fluid chamber retracts the first piston and the second piston downward, engagement balls of the switching mechanism connect the first piston to the housing to block the first piston from moving upward. On the other hand, when the pressure oil is discharged and the second spring advances the second piston upward by a predetermined stroke, the engagement balls connect the first piston to the second piston, whereby the urging force of the first spring is applied to the second piston via the first piston and the engagement balls.
However, since the second spring is mounted between the first piston and the second piston as described above, after the first piston and the second piston are connected and integrated with each other by the engagement balls, the urging force applied from the second spring to the second piston is received by the first piston, and thus only the urging force of the first spring advances the second piston upward. Therefore, there is room for improvement in increasing a force of advancing the second piston.
An object of the present invention is to allow a force of advancing a second piston to be greater than that in the conventional art.